Hydrocarbon distillates and residue-containing oils having characteristics which render them otherwise suitable for use as fuels for compression-ignition or diesel engines, or other atomising or vaporising-type burners, frequently have igition characteristics that render them unsuitable or only poorly suitable for such use. Fuels that have poor ignition characteristics, i.e., relatively high spontaneous ignition temperatures, exhibit an undesirably long ignition lag, between the time the fuel is injected into a zone of combustion and the time when the fuel ignites. In diesel engines, for example, a large ignition lag results in combustion of the fuel and the development of pressure over an improper portion of the crank angle period and piston stroke, resulting in knocking, rough engine operation, incomplete combustion in the combustion zone, power loss, and ultimately detriment to the engine.
To overcome these ignition or combustion problems, the fuel may be refined to produce a higher proportion of straight chain hydrocarbons similar to the original industry standard, cetane. The ignition quality of a diesel fuel is normally expressed in terms of its cetane number. The cetane number of a given fuel is defined as the percent proportion of cetane (a fast-burning C.sub.16 paraffinic constituent) in a .alpha.-methylnaphthalene (a slow-burning aromatic material) that will match the performance of the fuel at the same compression ratio in a standard test engine.
Various agents, including nitrates such as 2-ethylhexyl nitrate, have been used to improve cetane ratings. Certain nitrate esters are described for this purpose in EP-A-0146381. However, nitrates can give rise to NO.sub.x emissions in exhaust, and this is environmentally undesirable.
Peroxides have been widely used as free radical inhibitors (curing agents) for the polymer industry. Some have been proposed as cetane improvers. For example Kirsch et al, Combust. Flame 43 (1981) 11-21, describe the use of di-tert-butyl peroxide as well as certain nitrates for this purpose.
U.S. Pat. No. 3,468,962 describes the preparation of "peroxyacetals and peroxyketals" of the formula R.sup.1 OO--CR.sup.2 R.sup.3 --OR.sup.4 from alkylidenediperoxides of the formula R.sup.2 R.sup.3 C(OOR.sup.1).sub.2. The latter are in fact perketals as the term is now understood and as it is used herein. CR.sub.2 R.sub.3 can be a cyclaliphatic radical. The products are said to be useful as agents for improving the cetane number of gasolines.